Schafer U.S. Pat. Nos. 3,193,437 and 3,193,441, as well as Chant 3,867,221, illustrate the concept of an initially resilient and open-cell foam material impregnated with a thermosetting resin, used alone or in combination with reinforcing fibers, and wherein the foam layer is compressed under relatively low pressures such that the thermosetting resin substantially fills the cells of the foam and which resin is cured while maintaining the resilient foam layer in a compressed condition. The Chant and Schafer processes are particularly advantageous in constructing a relatively thin walled composite member of high strength and density. At the same time it is possible to utilize these processes to build up composite parts of considerable thickness. However, since in either of these processes it is necessary to compress the originally resilient open-cell foam material to between 1/2 and 1/4 of its original thickness in order to fill the open cells with resin, it is necessary to use either a very thick, initially resilient, open-cell foam material or several layers of such material, all impregnated with a thermosetting resin, in order to achieve a relatively thick part. The disadvantages of using such process alone to produce a part of thick cross section is that such part (1) becomes heavy relative to its strength and rigidity, (2) is costly due to the essentially solid resin matrix which extends throughout the part and (3) is a poor insulator as indicated by a low K factor.
While theoretically possible to make composite parts of relatively unlimited thickness using the molding process taught by Chant or Schafer, in practice parts with thicknesses beyond 7 to 8 mm (0.315 inch) lose certain of their important advantages including an important reduction in the strength-to-weight ratio as thickness increases.
The concept of a composite structure comprising a rigidified, closed-cell core element having a reinforced resin outer surface or casing is shown in U.S. Pat. No. 4,042,746 Hofer. In practice it is found in making a composite structure of the type taught in Hofer, the curing temperature for the resin which impregnates the resilient open-cell foam outer layer must be held in the range of 150.degree.-200.degree. F. in order to control the out-gassing of the air or gases trapped in the closed-cell element. Likewise, one or more of the side walls of the rigid core element are left unencapsulated to provide an exit path for any gases released or generated during the molding operation.
While satisfactory composite structures can be made in accordance with the teaching of the Hofer patent, the rate at which such parts can be made is limited by the relatively low resin curing temperatures which must be employed.
The use of small glass beads or spheres as an additive to a thermosetting resin used in a composite structure is shown in a companion U.S. Pat. No. 4,034,137 Hofer. In this Hofer patent the glass beads and fiberglass fibrils are added to a liquid resin which impregnates plural layers of open-cell, resilient foam for the purpose of decreasing the resin density and for increasing the bond strength between adjacent foam layers. U.S. Pat. No. 4,034,137 is concerned with enhancing laminar bond strength in relatively thin wall structures and does not relate to a closed-cell and relatively thick core element comprised of small hollow spheres imbedded in an organic resin which is in turn totally encased or encapsulated in a reinforced resin outer layer.
The method of making a reinforced composite structure utilizing a formable syntactic core element is shown in U.S. Pat. No. 4,025,686 Zion. In Zion a portion of the liquid resin of his uncured syntactic foam is forced through his reinforcing material to the exterior surface of his article. Zion also teaches that while his uncured resin flows outwardly, the microspheres are too large to pass through the reinforcing material and thus remain in the core of the article. Several problems result from the method taught by Zion. First, as Zion's resin flows out of his uncured syntactic foam core element to provide a resin-rich outer surface, the proportion of resin to microspheres is reduced and, in turn, reduces the strength of the cured core portion of his composite. In other words, the less resin in the core element in proportion to microspheres, the weaker the core element portion of the composite and the greater the likelihood of delamination or other structural failure through the core element.
Next, in flowing outwardly during molding, Zion's uncured resin will likewise cause his microspheres to flow outwardly toward and to concentrate against his fibrous reinforcing material. Since his microspheres cannot pass through his reinforcing layer, said layer acts as a sieve having a high concentration of microspheres at the interface of the core element and the reinforcing layer. Such high concentration of microspheres inevitably decreases the laminar strength of the resultant composite along such interface.
On the other hand, Applicant's method, while providing an initially amorphous or formable core element, combines such core with super-adjacent, resin-impregnated, compressible and reinforced outer layers to prevent the flow of resin out of the core element and to maintain a substantially uniform distribution of hollow spheres and resin throughout the core of the composite structure.